1. Field of the Invention
The present invention relates to electric motors and, in particular, relates to an electric motor having a rotor-embedded sensor. In a particularly preferred embodiment, the present invention relates to an induction motor having a rotor-embedded sensor that measures the current through one or more rotor windings.
2. Description of Related Art
Electric motors comprise a stator and a rotor that is capable of rotating relative to the stator. For precise motor control, it is generally desirable to know as much as possible regarding motor operating conditions, including conditions that are parameters which can be measured and assigned a value. Consequently, a wide variety of sensors have been provided that are useful for sensing motor operating conditions. For example, position and velocity transducers have been provided that can be mounted to an output shaft of a motor so that the angular position and/or angular velocity of the motor may be measured. As well, current sensors have been provided that can be coupled to the stator windings so that the current flowing the stator windings may be measured.
However, few if any sensors have been provided that are capable of directly measuring conditions associated with the rotor. For example, no known sensor has been provided that is capable of measuring current in the rotor windings of an induction motor. In an induction motor, current is induced in the rotor windings magnetically and, as a result, there are no electrical connections between the rotor windings and the remainder of the motor that can serve as the basis for performing such measurements. Thus, the rotor current cannot be directly measured using existing approaches.
Nevertheless, the ability to perform such measurements would be extremely useful. The magnitude of the current in the rotor windings of an induction motor is indicative of the torque produced by the motor shaft. Additionally, the frequency of the current in the rotor windings is the slip frequency, which can be used to determine the speed of the motor shaft. Finally, knowledge of the phase of the rotor current could be used to synchronize the phase of the current that is applied to the stator windings with the angular position of the rotor. Thus, the ability to measure rotor current would be extremely useful because it would enhance the ability to precisely control the induction motor.
Beyond motor control, the ability to measure rotor current would also be extremely useful for other reasons. For example, the rotor windings of an induction motor are generally formed of solid metal bars that are subject to fatigue and breaking. Currently, broken rotor bars are diagnosed by the acoustical noise and/or the excessive heat that is generated when a rotor bar breaks. However, it is often not possible to diagnose breaking in this way until several rotor bars windings are affected, by which time the motor must be replaced immediately. Therefore, it would be extremely useful to be able to measure the rotor current as a way of detecting broken rotor bars.
Other parameters that are useful to measure include rotor temperature, rotor airgap, rotor flux and rotor torque. Measuring these parameters is useful not only in conjunction with induction motors, but also in conjunction with all types of motors.
Accordingly, what is needed is a way to measure these and other rotor-associated operating conditions.